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Stellar spectra captured from ground based equipment needs corrections to remove atmospheric spectral noise. Is there an Internet site that shows specific amplitude and wavelength differences between an observed spectrum and a corrected stellar spectrum?

The Dec, 2011 issue of Astronomy, page 51 shows the interference of Earth's atmospheric spectra, but the figures do not have amplitude and wavelength data.

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4 Answers 4

I think the question is a little misleading. If you are taking long exposures of an stellar spectrum then the only "noise from the atmosphere" you have to account for are absorption, refraction and dispersion, which is basically what Andrew answered (for more details see the works of Vacca, Cushing or Rayner, 2003, where they plot the spectrum that you are asking for I think).

However, if you are actually taking short exposures and want to correct for the real noise that produces changes on stellar spectra because of climate changes, you are probably talking of red noise, which have been a concern for years for photometry and is today being used in spectroscopy (see the work of Pont et al, 2005 for a short introduction).

If no answer is useful for you, maybe you'll have to give us details in what you want (and please, if you saw this on a magazine, post the link to the article or to some preprint that we could see in order to help you. For example, I'm not actually subscribed to the Astronomy magazine so I really don't know what figure caught your mind).

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" the only "noise from the atmosphere" you have to account for absorption, refraction and dispersion" - are you saying that there is no such thing as atmospheric emission lines? Because in that case, you are wrong. –  Thriveth Sep 15 '13 at 21:13
    
Stimulated emission counts as absorption with a negative absorption coefficient ;-). I suppose the abundance of every atom is too low to produce a significant (measurable?) amount of spontaneous emission. –  Néstor Sep 17 '13 at 19:53
    
Ah sure. I guess by "noise" was meant "contamination", not necessarily "spontaneous emission"... –  Thriveth Sep 17 '13 at 20:59
    
That's what I understood at least. –  Néstor Sep 17 '13 at 22:51

The UVES Sky line atlas (or UVES Sky Emission Spectrum) is a very high resolution and high quality map of the wavelengths of sky emission lines. I have in my Master's Thesis a description of how these lines can be used to calibrate the pixel-to-wavelength conversion of a spectrum.

Unfortunately, such an atlas isn't worth much when it comes to removing these lines, as their relative intensity change with weather, water content etc. of the atmosphere on the given day. To get rid of them, one has to either perform some (rather painstaking) mathematical modelling, or use this trick at the time of observation:

You can, right before or right after observation, point the spectrograph slit or aperture to a dark spot in the sky right next to the object you were observing. Far enough that you don't have scattered light from the object spilling in, but so close that you are basically pointing at the same angle in the sky - and make an "empty" exposure of the sky. This will basically give you a snapshot of the atmospheric emission, and you can now subtract this image from the observation (making sure they are exposed for exactly equally long, possibly applying a correction factor to one of them). This should leave you with a spectrum cleaned of atmospheric noise.

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You may find the information in this post enlightening (no pun intended): http://scienceblogs.com/startswithabang/2012/02/defeating_hubble_from_the_grou.php

It's about using http://en.wikipedia.org/wiki/Adaptive_optics to cancel out atmospheric interference.

This isn't necessarily an answer to your question but I can't comment yet.

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The basic of atmospheric extinction, i.e. the part that is uniform worldwide, are explained here: http://www.asterism.org/tutorials/tut28-1.htm (basically geometric scattering)

The details will vary with time and geographic location- sodium lamp light pollution, molecular absorption lines, aerosols in the air, volcanic ash, the whole gamut. Really doing it right is a matter of ongoing research.

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Andrew, your answer addresses atmospheric extinction rather than atmospheric spectra that are the noise for stellar spectra observations from Earth-based instruments. As mentioned in my question, I was looking for a site illustrating a stellar spectrum as observed combined with atmospheric spectra (noise) and as would be seen above the atmosphere. –  Mike Nov 20 '11 at 18:38
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By the way, the atmospheric scattering reference in your response was written by me several years ago as a tutorial for my astronomy club. –  Mike Nov 20 '11 at 22:55

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